drm/nouveau/fbcon: take runpm reference when userspace has an open fd

We need to do this in order to prevent accesses to the device while it's
powered down.  Userspace may have an mmap of the fb, and there's no good
way (that I know of) to prevent it from touching the device otherwise.

This fixes some nasty races between runpm and plymouth on some systems,
which result in the GPU getting very upset and hanging the boot.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org